An image forming apparatus used as a printer, facsimile machine, copier, or multi-functional device thereof may use a fixing member to fix a toner image on a recording medium. Such a fixing member may be a fixing belt, film, or roller, for example.
For example, a conventional image forming apparatus may include a fixing belt having an endless shape, a plurality of roller members for extending and supporting the fixing belt, a heater provided inside one of the plurality of roller members, and a pressure roller serving as a pressure member. When the fixing belt is heated by the heater via the one roller member, a recording medium having a toner image thereon is conveyed to a nip formed between the fixing belt and the pressure roller. When heat and pressure are applied to the recording medium at the nip, the toner image is fixed on the recording medium.
Another conventional image forming apparatus uses an on-demand fixing device operable with a relatively short warm-up time. Such an on-demand fixing device may include an endless fixing film serving as a fixing member, a pressure roller serving as a pressure member, and a heater, for example, a ceramic heater. The heater is disposed at an inner surface side of the fixing film to heat the fixing film. The heater also forms a nip by abutting the pressure roller via the fixing film. When a recording medium is conveyed to the nip, heat and pressure are applied to a toner image on the recording medium at the nip and thus the toner image is fixed on the recording medium.
The belt-type fixing device as described above is relatively advantageous in increasing its operation speed compared to a roller-type fixing device. However, such belt-type fixing device may not be so effective to shorten so called “warm-up time,” which is the time required to reach a temperature suitable for operation, and “first print time,” which is the time required for completing a print operation starting from receipt of a print request, printing, and ejection of a printed medium.
On the other hand, for the on-demand fixing device using a fixing film, the warm-up time and first print time may be shortened by reducing a heat capacity thereof, thereby reducing the size thereof. However, in the on-demand fixing device, only the nip of the fixing film is locally heated while the other portions may be not sufficiently heated. As a result, during rotation, the fixing film may have a relatively low temperature at the sheet entry side of the nip, thereby causing defective fixing. Further, when the fixing device is operated at higher speed, the fixing film is also rotated at higher speed, thereby further facilitating heat dissipation at the portions other than the nip.
In order to prevent such defective fixing, a tubular metal heat conductor may be provided so as to face an inner surface of an endless fixing member. By directly or indirectly heating the metal heat conductor, the entire portion of the fixing member may be sufficiently and uniformly heated.
However, in such a configuration, the tubular metal heat conductor might be undesirably deformed by pressing force, which is applied at the nip from the pressure member to the metal heat conductor. Because the tubular metal heat conductor has been preferably formed thinner in order to enhance the heat efficiency of the fixing member, such a configuration may more easily cause defective fixing.
Deformation of the metal heat conductor may cause severe rubbing against the inner surface of the fixing member at the deformed portion. As a result, the inner surface of the fixing member may be unevenly worn out, thereby deteriorating the durability thereof. Alternatively, a driving torque of the fixing member may be increased, thereby causing slip of the fixing member and jam of a recording medium at the nip.
The deformation of the metal heat conductor may also reduce its contact area with the pressure member on a central portion in a width direction thereof. As a result, the nip width of the deformed portion may become smaller than the nip width of each end portion of the metal heat conductor. In such state, the amount of heat which the metal heat conductor applies to the recording medium is different between the central portion and each end portion. Consequently, cold offset may be caused at a central portion of an output image, while hot offset may be caused at end portions thereof. Thus, a relatively large difference in glossiness may occur between the central portion and the respective end portions. Further, an extremely large deviation in nip width may occur between the central portion and each end portion. Such deviation may significantly affect the output image and, in some cases, might cause cockling in the recording medium.